Frequency Comparison of Point-Contact and Junction Transistors

Author: Leonard Krugman

At this point, a brief explanation of why the point-contact transistor is capable of a higher operating frequency than the junction type is in order. The high frequency effects on the equivalent circuit parameters are essentially the same for both types. Actually, the major difference is in the mechanics of conduction.

Point-contact transit time is determined primarily by the field set up by the collector current. In equation form, where S is the point spacing in centimeters, μ is the hole mobility in, ρ is the germanium resistivity in ohm-cm, and I is the collector current in amperes. Typical transistive values are S = .003cm,

ρ = 12 ohm-cm, and I_c - 3 ma, for which T = 1,570 μμsecs. Ignoring all other factors, this limits the upper frequency response of the point-contact transistor to about 600 megacycles.

In the junction transistor, movement of the current carriers is primarily by diffusion, and is not appreciably affected by the electrode potential fields. In equation form, where T is the diffusion time through the base layer, W is thickness of the base layer in centimeters, and D is the diffusion constant in cm²/sec. Typical values for a P-N-P transistor are W = 2 × 10^-3 cm and D = 33 cm²/sec (for an N-P-N type, D is about 69 cm²/sec), for which T = 0.121 μsecs. Ignoring all factors but the diffusion time, the upper frequency for this typical P-N-P type is approximately 8 me, and for the N-P-N type about 16 mc.